1. Field of the Invention
The present invention generally relates to projection television receivers, and particularly relates to cooling liquids for cooling a projection cathode ray tube in a projection television receiver.
2. Description of the Background Art
Referring to FIG. 1, a conventional cooling system for a projection television receiver will be described.
In FIG. 1, a projection cathode ray tube 21 and a glass plate 22 are bonded with each other through a silicon group adhesive agent 24. A radiator 23 is interposed between the projection cathode ray tube 21 and a glass plate 22 such that a space is formed between the projection cathode ray tube 21 and a glass plate 22. The space is filled with cooling liquid A poured through a screw hole 23a opened to the radiator 23. The screw hole 23a is sealed by means of a packing 26 and a screw 25 after the cooling liquid has been inserted. The reference numeral 27 designates a lens.
When a signal is applied to the projection cathode ray tube 21, the front face of the cathode ray tube 21 is heated up. If there were no cooling system, as described above, the temperature would reach about 120.degree. C. at the center portion of the front face and about 72.degree. C. at the periphery of the same, and the temperature at the fluorescent surface of the projection cathode ray tube 21 could reach values higher by 20.degree. C. than those at the front face.
Accordingly, if the above-mentioned space is filled with the cooling liquid A as described above, convection is effected in the cooling liquid A as shown in FIG. 2, in such a manner that the heat at the center portion of the projection cathode ray tube 21 is transmitted to the cooling liquid A and the heated cooling liquid A moves up to the radiator 23. Thus, the heat of the cooling liquid A is absorbed to be exhausted outside by the radiator 23. The thus cooled liquid A in the radiator 23 then moves down again by thermosiphoning into the space.
As the cooling liquid, a mixture of ethylene glycol and water is used with a ratio of the former to the latter of about 4:1. The physical properties of the cooling liquid are as shown in the following Table.
Since the refractive index of the cooling liquid A is 1.41, the reflection factor (R.sub.1, R.sub.2) at the boundary in the structure as shown in FIG. 1 is as follows. ##EQU1## where 1.54 is a value of the refractive index of the glass. Thus, the total reflection factor at the boundaries between the cooling liquid A and the projection cathode ray tube 21, and between the cooling liquid A and the glass plate 22 reaches 0.38%. This has been a cause of a marked decrease in contrast in conventional projection television receivers.
Since the cooling liquid A contains 20% of water, the vapor pressure reaches 0.47 atm at a temperature of 100.degree. C., and this vapor pressure acts directly on the projection cathode ray tube 21, as well as the glass plate 22, with a risk of damage of those components.
TABLE ______________________________________ Cooling liquid Cooling liqud of the Invention of the Prior Art Mixture of Mixture of ethylene glycol ethylene glycol and glycerol and water (30% by weight) (20% by weight) ______________________________________ Refractive index 1.444 1.41 Boiling point 205.degree. C. 126.degree. C. Solidifying point -75.3.degree. C. -45.degree. C. (Melting point) Flash point 132.degree. C. 118.degree. C. Thermal 6.47 .times. 10.sup.-4 7.3 .times. 10.sup.-4 conductivity cal/cm .multidot. s .multidot. deg cal/cm .multidot. s .multidot. deg Vapor pressure 80.degree. C. 0.003 (atm) 0.22 (atm) 100.degree. C. 0.011 (atm) 0.47 (atm) 120.degree. C. 0.026 (atm) 0.94 (atm) ______________________________________
If the glass plate 22 is replaced by a plastic lens, the plastic material of the lens may absorb the water and become cloudy white because the cooling liquid contains a large quantity, about 20%, of water. Accordingly, a plastic lens cannot be used at the point where the lens comes into contact with the water. Therefore if a plastic lens is used, it must be disposed in front of a heavier glass plate 22 preventing any substantial reduction in weight or cost.
It is an object of the present invention to provide a liquid lens for a projection television receiver, in which the reflection factor at boundaries can be made smaller to make the contrast higher, in which the vapor pressure can be low to eliminate the risk of damage of the projection cathode ray tube or the like, and in which a plastic lens can be used and the plastic lens can constitute a direct cooling surface. Thus, it is possible to reduce the weight, as well as the cost of the device.
Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings.